The present invention relates generally to an apparatus for mounting printing plates and, in particular to an apparatus for mounting flexible printing plates on press cylinders.
In one form of the printing process, printing is effected by photopolymer or rubber printing plates mounted on cylinders, the paper to be printed being impressed on the inked printing plate. The cylinder on which the printing plates are mounted is generally called the plate or printing cylinder. The quality of a printing job depends, in a large measure, on the care in which pre-press preparations are carried out. Plate-mounting, color registration and proofing are effected off the press by means of commercially available mounting-proofing machines designed for this purpose.
The mounting of photopolymer or other printing plates onto plate cylinders for printing therefrom requires a high degree of accuracy in the alignment thereof. The image must be square and in register on the cylinder in order to print square and in register on the work. In the printing of colors or in the superimpression of images, the various colors or images are added sequentially. Accordingly, it is important that in each case the printing plate which is adding the successive color or image be synchronized with the preceding plate or plates so that the colors or images are accurately superimposed. To arrange these plates in the exact predetermined relation to one another requires that their angular as well as their transverse position on the printing plate support means be accurately determined. In the prior art this synchronizing has been performed by mechanical methods and apparatus which are complicated in their implementation and easily subject to inaccuracies. In addition, in the past the synchronizing of the printing plates has been done while they were in position in the printing press. This is not only inconvenient and presents difficult working conditions, but also the printing press is out of operation during this time.
One common method to effect the alignment of the plates with respect to the print cylinder involves the drawing of a line around the print cylinder. This line is then aligned by eye with a longitudinal line along the length of the photopolymer or other print plate. This method is relatively accurate but can be extremely time consuming for the operator. This leads to delay between print runs and is costly with respect to the time lost between such runs.
Alternatively, there is commercially available a device to aid in the alignment of photopolymer or printing plates onto the print cylinder. The print cylinder is placed in a fixed relationship to the device and the plate is laid upside down on a clear glass top. By means of a series of mirrors having lines drawn thereon, the plate is aligned relative to the print cylinder. However, this device is also relatively time consuming and the required accuracy is not achieved. There is only a one-to-one relationship between the eye of the operator and the device assisting in the alignment which can lead to errors of up to one millimeter. These errors are unacceptable where accurate printing is required. This device is generally only acceptable for the alignment of printing plates with respect to one another rather than with respect to the print cylinder.
These machines, which usually make use of an optical mounting system, make it possible to mount the plates on plate cylinders to effect exact color registration, a procedure essential to the maintenance of both quality and economy in all flexible plate printing operations. Pre-proofing is, in many respects, the most important of all pre-press preparations, for it not only indicates the appearance of the final reproduction, but it also affords means to check the mounting of the plates for color sequence, spacing requirements, layout and gear size, as well as copy and color separation.
Mounting-proofing machines have been provided with a proofing cylinder (sometimes called the impression cylinder) which cooperates with the printing cylinder, the proofing cylinder making contact with the printing plates on the printing cylinder and rotating concurrently therewith to print a proof on a sheet secured to the proofing cylinder. In commercial machines of the type heretofore known which make use of optical mounting techniques, the proofing or impression cylinder is supported for rotation in a fixed position, whereas the printing cylinder is moveable, usually in a vertical direction, from a mounting state in which it is retracted relative to the proofing cylinder to a proofing state in which it is in engagement therewith.
The proofing and printing cylinders are mechanically intercoupled, whereby rotation of the proofing cylinder causes the printing cylinder to rotate. When the diameter of the proofing cylinder is the same as the printing diameter of the printing cylinder (i.e., the diameter of the printing cylinder plus the thickness of the printing plates thereon). then a one-to-one relationship exists therebetween. However, printing cylinders are manufactured in a range of diameters for printing different print lengths. Therefore, it has been necessary to adjust the phase relationship between the printing and proofing cylinders to accommodate the differences between the cylinder diameters. For adjusting this phase relationship for different printing cylinder diameters, a relatively complex mechanism is required in existing types of mounting-proofing machines.
Another drawback of existing types of mounting-proofing machines is their limited capacity to handle printing cylinders of different diameter. With machines of the type heretofore known, the capacity of the machine is restricted to a range of printing cylinder diameters extending from about ninety-five percent of the diameter of the proofing cylinder down to about twenty-five or thirty percent thereof, or approximately four to one. Moreover, since in existing structures, the proof forces imposed at contact are eccentrically opposed, the structures required to accommodate these magnified forces are too large to permit smaller sizes of printing cylinders to fit the machine.